Re: [PATCH] Enhance perf to collect KVM guest os statistics fromhost side

[Ingo Molnar]

* Avi Kivity <avi@xxxxxxxxxx> wrote:

> On 03/16/2010 11:53 AM, Ingo Molnar wrote:
> >* Avi Kivity<avi@xxxxxxxxxx>  wrote:
> >
> >>On 03/16/2010 09:24 AM, Ingo Molnar wrote:
> >>>* Avi Kivity<avi@xxxxxxxxxx>   wrote:
> >>>
> >>>>On 03/16/2010 07:27 AM, Zhang, Yanmin wrote:
> >>>>>From: Zhang, Yanmin<yanmin_zhang@xxxxxxxxxxxxxxx>
> >>>>>
> >>>>>Based on the discussion in KVM community, I worked out the patch to support
> >>>>>perf to collect guest os statistics from host side. This patch is implemented
> >>>>>with Ingo, Peter and some other guys' kind help. Yang Sheng pointed out a
> >>>>>critical bug and provided good suggestions with other guys. I really appreciate
> >>>>>their kind help.
> >>>>>
> >>>>>The patch adds new subcommand kvm to perf.
> >>>>>
> >>>>>   perf kvm top
> >>>>>   perf kvm record
> >>>>>   perf kvm report
> >>>>>   perf kvm diff
> >>>>>
> >>>>>The new perf could profile guest os kernel except guest os user space, but it
> >>>>>could summarize guest os user space utilization per guest os.
> >>>>>
> >>>>>Below are some examples.
> >>>>>1) perf kvm top
> >>>>>[root@lkp-ne01 norm]# perf kvm --host --guest --guestkallsyms=/home/ymzhang/guest/kallsyms
> >>>>>--guestmodules=/home/ymzhang/guest/modules top
> >>>>>
> >>>>Excellent, support for guest kernel != host kernel is critical (I
> >>>>can't remember the last time I ran same kernels).
> >>>>
> >>>>How would we support multiple guests with different kernels? Perhaps a
> >>>>symbol server that perf can connect to (and that would connect to guests in
> >>>>turn)?
> >>>The highest quality solution would be if KVM offered a 'guest extension' to
> >>>the guest kernel's /proc/kallsyms that made it easy for user-space to get this
> >>>information from an authorative source.
> >>>
> >>>That's the main reason why the host side /proc/kallsyms is so popular and so
> >>>useful: while in theory it's mostly redundant information which can be gleaned
> >>>from the System.map and other sources of symbol information, it's easily
> >>>available and is _always_ trustable to come from the host kernel.
> >>>
> >>>Separate System.map's have a tendency to go out of sync (or go missing when a
> >>>devel kernel gets rebuilt, or if a devel package is not installed), and server
> >>>ports (be that a TCP port space server or an UDP port space mount-point) are
> >>>both a configuration hassle and are not guest-transparent.
> >>>
> >>>So for instrumentation infrastructure (such as perf) we have a large and well
> >>>founded preference for intrinsic, built-in, kernel-provided information: i.e.
> >>>a largely 'built-in' and transparent mechanism to get to guest symbols.
> >>The symbol server's client can certainly access the bits through vmchannel.
> >Ok, that would work i suspect.
> >
> >Would be nice to have the symbol server in tools/perf/ and also make it easy
> >to add it to the initrd via a .config switch or so.
> >
> >That would have basically all of the advantages of being built into the kernel
> >(availability, configurability, transparency, hackability), while having all
> >the advantages of a user-space approach as well (flexibility, extensibility,
> >robustness, ease of maintenance, etc.).
> 
> Note, I am not advocating building the vmchannel client into the host 
> kernel. [...]

Neither am i. What i suggested was a user-space binary/executable built in 
tools/perf and put into the initrd.

That approach has the advantages i listed above, without having the 
disadvantages of in-kernel code you listed.

Thanks,

	Ingo
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